A tankie is, broadly, someone who wants to effect left-wing ideology using authoritarian methods. It originally referred to those who defended the USSR using tanks to suppress the Hungarian Revolution, but it could be aptly used to describe those who defend China’s actions in Tiananmen Square. It’s rightfully used as a perjorative, since authoritarian enforcement is antithetical to leftism, particularly communism.
Tankies are hypocrites who didn’t understand their self-proclaimed ideologies. If someone’s idea of communist praxis is lining up dissenters for the firing squad, you’re dealing with a tankie.